248 E. C. ANDREWS. 



or cup-shaped heads. Such " steps" will pursue, or lag 

 behind, each other according as the local stream velocity 

 is relatively increased or retarded. In other words a 

 channel declivity with interrupted grade may have been 

 formed by a stream. Another constant but greater stream 

 volume may have been superimposed and the new fluctu- 

 ations of channel grade express its variable corrasive power. 



(iv) Weak structures enclosed by strong ones. 



Weak structures do not imply increase of stream velocity 

 but the general appearance after corrasion in structures of 

 varying strength is somewhat similar to that experienced 

 by homogeneous structures when acted on by accelerated 

 stream velocity locally, A stream thus will pick out an 

 area of weak structures on a declivity and excavate therein 

 a "step" — basined or not basined — with a strong "tread" 

 at its head. The question of basin history upon reduction 

 of stream volume will be dealt with now. 



O. History of some channel basins and "over deepened' 1 '' 

 channels upon reduction of stream volume. 



Let us now consider the effect of reducing the stream 

 volume under consideration. It will be well to consider 

 two stages in this process : — 



(i) A reduction to half, or a quarter, of the former volume. 



(ii) A very great reduction in volume, say to the 

 "drought " stage. 



(i) Reduction to half, or a quarter, of the former volume. 



The stream volume is now lessened yet its size when 

 considered absolutely may be still very large. Pressure 

 in this case is still so great that flowage is marked and 

 velocity is pronounced on declivities. There are here two 

 stages to be considered, firstly, one of channel grade read- 

 justment, and secondly, this readjustment stage followed 

 by a corrasive stage (with the production of forms adapted 

 to the requirements of the new stress). That this is so 





